Positioning Architecture
FIG. 1 depicts an example of a LTE positioning architecture. Positioning in Long Term Evolution (LTE) is supported by the architecture in FIG. 1, with direct interactions between a user equipment (110) and a location server (120), such as Evolved-Serving Mobile Location Centre (E-SMLC), is via a LTE Positioning Protocol (LPP). Moreover, there are also interactions between the location server and the eNodeB (130) via a LPP protocol, to some extent supported by interactions between the eNodeB and the UE via the Radio Resource Control (RRC) protocol.
The following positioning techniques are considered in LTE, e.g. in Third Generation Partnership Project (3GPP) Technical Specification (TS) 36.305:                Enhanced Cell ID. Essentially cell ID information to associate the UE to the serving area of a serving cell, and then additional information to determine a finer granularity position.        Assisted GNSS. GNSS information retrieved by the UE, supported by assistance information provided to the UE from E-SMLC        OTDOA (Observed Time Difference of Arrival). The UE estimates the time difference of reference signals from different base stations and sends to the E-SMLC for multi-lateration.        Uplink Time Difference of Arrival (UTDOA). The UE is requested to transmit a specific waveform that is detected by multiple location measurement units (e.g. an eNB) at known positions. These measurements are forwarded to E-SMLC for multi-lateration.        
FIG. 2 depicts OTDOA position estimation based on multi-laceration of the RSTD measurements. The Observed Time Difference Of Arrival (OTDOA) is a UE-assisted method, in which the UE measures the Time Of Arrival (TOA) of specific Positioning Reference Signals (PRS) from multiple eNBs, and computes the relative differences, i.e. Reference Signal Time Differences (RSTD). The RSTD are quantized and reported via LPP to the E-SMLC together with an accuracy assessment. Based on known positions of eNBs and their mutual time synchronization, it is possible for the E-SMLC to estimate the UE position from the RSTD and covariance reports using multi-lateration. The accuracy depends on the radio conditions of the received signals, number of received signals as well as the deployment, which means that it will vary spatially. FIG. 2 illustrates the multi-lateration in OTDOA while considering eNB1 as the reference cell.
RSTD Measurement Reporting
The UE estimates the time of arrival of a reference signal from a detected reference cell and other detected cells based on assistance information received from E-SMLC. Then, the UE computes the Reference Signal Time Difference (RSTD) of each reference signal for all detected cells in relation to time of arrival of the reference signal from the detected reference cell. The RSTD is subject to a quantization with a resolution of 1 Ts for RSTDs within ±4096 Ts, and 5 Ts otherwise (1 Ts=1/(15000×2048) sec is the LTE basic time unit). The UE then sends a RSTD measurement report, including the quantized RSTD, to the location server.
In addition, the UE estimates the RSTD measurement quality and reports, e.g. in a RSTD quality assessment report, the uncertainty via a range:[nR,(n+1)R−1],
Where the reporting resolution is R={5, 10, 20, 30} meters, and n is an index to indicate the value range within which the RSTD uncertainty is estimated to be.
Problems with the Existing Solution
The accuracy of the position estimation depends significantly on the quality and accuracy of the measured RSTDs sent from the UE to the E-SMLC.
Need for Finer RSTD Measurement Reporting Resolution
There are several indications that current UE receiver implementations can estimate the RSTD at a finer resolution than the current RSTD measurement reporting resolution of 1 Ts, which corresponds to 9.8 meters. For example, baseline evaluations indicate that performance is much restricted by the RSTD quantization resolution. Furthermore, there are also academic results available showing that the Cramer-Rao Lower bound for timing estimation based on LTE PRS signals, as well as estimators almost attaining the bound, are well below the RSTD resolution for intermediate to good radio conditions, e.g. in terms of Signal-To-Noise-Ratios (SNRs).
In contributions to RAN1 #80, there were proposals to widen the bandwidth of the Positioning Reference Signal to 20 MHz. That would provide more positioning symbols within each sub-frame, and thereby enable better time synchronization performance. Moreover, also the current mapping of the PRS sequence on time/frequency resource elements causes some auto-correlation issues that can lead to time synchronization errors with some receiver implementations. This is also addressed as a potential area for improvements. In theory, these improvements can provide significant benefits. However, these benefits may be completely suppressed by the RSTD measurement reporting limitations.